Shielding plate in plasma for uniformity improvement

ABSTRACT

An apparatus comprising a plasma chamber containing a plasma for a plasma-assisted material process upon a substrate; a shielding plate within the plasma chamber to actively direct ion flux to desired areas of the substrate; and a supporting structure to support the shielding plate within the chamber is disclosed.

FIELD OF THE INVENTION

[0001] The field of the invention relates generally to a plasma-assistedmaterial process, and more particularly to control of process uniformityacross substrate.

BACKGROUND OF THE INVENTION

[0002] Capacitively coupled plasma (CCP) reactors and high densityplasma (HDP) reactors, such as inductively coupled plasma (ICP) reactorsand electron cyclotron resonant (ECR) plasma reactors, have been widelyapplied in the semiconductor industry for plasma assisted materialprocess, such as plasma enhanced chemical vapor deposition (PECVD) andplasma etch. A conventional plasma reactor, such as CCP, usuallyconsists of two parallel plate electrodes in a chamber. The reactivenature of the discharged gas in the chamber is sustained due to theradio frequency (RF) voltage on the two electrodes. The typical pressurein the chamber ranges from 10⁻³-10 Torr. High voltage on the electrodescauses ion bombardment on the surface of a substrate.

[0003] HDP reactors, such as those of the ICP type, consist of two setRF coils located outside the plasma chamber. RF power is provided to thechamber by an inductive magnetic field. It is not necessary to have anelectrode in the HDP reactor. In general, by applying RF bias voltage tothe substrate one can independently control the ion bombardment energyin HDP.

[0004] Plasma-assisted material processes are generally carried out in aplasma chamber or reactor. To control the reaction rate across a reticleor wafer, the state of the art approach provides for the end user toadjust operational parameters. For example, the radio frequency (RF)power of the coil or neutral gas pressure may be adjusted. Additionally,the mass flow rate may be adjusted.

[0005] However, modification of the above parameters still yields adisproportionate etching or deposition rate on the surface of thesubstrate, i.e. sometimes significantly higher rates at the center ofthe substrate than at the edges. Thus, such modifications can yield, atbest, only about a 2-3% uniformity across the substrate. Although thisrepresents an improvement over processes performed without suchmodifications, this level of uniformity still does not meet thespecifications for certain applications, such as for etching aquartz/chromium reticle.

[0006] With state of the art systems, the end user cannot do anythingmore to improve uniformity without sacrificing other qualities such asetch selectivity. The end user cannot make structural modifications tothe reactor hardware itself, such as, adjustment of coil configuration,chamber size, and pedestal position. Such modifications could producethe desired improvements, but modifications to reactor hardware can onlybe made by the vendor. Thus, if heightened uniformity is to be achievedby the end-user, it should make use of conventional etching equipment.

[0007] There is thus a need for a means of attaining betterplasma-enhanced material process uniformity using conventionalequipment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings in which likereferences indicate similar elements.

[0009]FIG. 1 is an illustration of a cross-sectional view of oneembodiment of a shielding plate in a plasma reactor, in accordance withthe present invention.

[0010]FIG. 2 is an illustration of a top-view of two embodiments ofshielding plates, in accordance with the present invention.

[0011]FIG. 3 is an illustration of a cross-section of plasma densitiesfor various embodiments of plate usage, in accordance with the presentinvention.

[0012]FIG. 4 is an illustration of radial ion flux rates for the variousembodiments of plate usage in FIG. 3, in accordance with the presentinvention.

[0013]FIG. 5 is an illustration of a cross-section of plasma potentialsfor the various embodiments of plate usage in FIG. 3, in accordance withthe present invention.

[0014]FIG. 6 is an illustration of a cross-section of one embodiment ofa critical dimension control process, in accordance with the presentinvention.

[0015]FIG. 7 is a flow diagram for one embodiment of a method foroptimizing plasma-assisted material process uniformity, in accordancewith the present invention.

[0016]FIG. 8 is a flow diagram for one embodiment of a method foroptimizing a critical dimension control process, in accordance with thepresent invention.

DETAILED DESCRIPTION

[0017] A shielding plate is placed in plasma for plasma-assistedmaterial process uniformity improvement. An apparatus comprising aplasma chamber containing a plasma for a plasma-assisted materialprocess upon a substrate; a shielding plate within the plasma chamber toactively direct ion flux to desired areas of the substrate; and asupporting structure to support the shielding plate within the chamberis disclosed.

[0018] The apparatus provides an effective way to control ion flux andoptimize the uniformity of plasma-assisted material processes, such asetching or deposition, across the surface of a substrate. It can alsoprovide a way to localize reactions on the substrate. It can alsominimize the macro-loading effect due to non-uniform accumulation ofetching by-products, which may cause substrates to become unbalanced. Itmay also be favorable to current critical dimension control processes.The apparatus can result in cost savings in the material processes.

[0019] The apparatus comprises a shielding plate within a plasma reactorto regulate the material process. The term plasma-assisted materialprocess will be used interchangeably with the terms plasma-assistedetching process and plasma enhanced chemical vapor deposition (PEVCD).Further, these terms may be abbreviated to etching and deposition. Inother words, the apparatus is not limited to etching processes. The termplasma reactor will be used to mean an inductively coupled plasmareactor or other plasma reactor. The plate can regulate ion flux acrossthe substrate during an etch process. Ion flux is the quantity of ionsdiffusing through and perpendicular to a unit cross-sectional area(i.e., the substrate) per unit time. An ion is an atom or molecule witha positive charge because electrons have been removed or a negativecharge because electrons have been added. Negative ions do not diffuseto the chamber wall and substrate because of the electrostatic field inplasma. A plasma is a discharged gas in which some individual atoms areionized though the total number of positive and negative charges isequal, maintaining an overall electrical neutrality.

[0020] The etch rate is the rate of controlled surface erosion upon asubstrate. In one embodiment, the substrate subject to plasma-assistedetching is a reticle. In another embodiment, the substrate is a siliconwafer. The terms substrate, reticle, and wafer will be usedinterchangeably. In other embodiments, the substrate is polysilicon,aluminum or copper. Although the detailed mechanism of why ions playsuch a critical role in the etching process is still unclear, it isbelieved that etch rate is proportional to both ion flux and activespecies flux. Therefore, etch uniformity can be optimized by controllingthese fluxes. Similarly, ion flux is proportional to the rates of PECVD.

[0021] The radius and the location of the plate may depend upon the typeof etch process performed. By adjusting the size, location and geometryof the plate within the plasma reactor, the ion flux can be activelycontrolled.

[0022]FIG. 1 is an illustration of a cross-sectional view of oneembodiment of a shielding plate in a plasma reactor, in accordance withthe present invention. Shielding plate 110 is supported by support 120.Support 120 sets upon pedestal 150 within plasma reactor 140. The termplasma reactor will be used interchangeably with plasma chamber. In oneembodiment, the plate 110 is supported by three supports 120. In analternate embodiment, shielding plate 110 is suspended from above thereticle 130. Alternatively, shielding plate 110 is located to the sideof reticle 130. Reticle 130 sets upon pedestal 150. Neutral, reactivegas, such as but not limited to CF₄ or CHF₃ enters chamber 140 throughgas inlet 160. When the neutral gas enters the strong electrical fieldwithin chamber 140, it becomes a plasma. The plasma fills chamber 140and ions flow anisotropically through the plasma sheath and hits thewafer vertically. Anisotropic flow varies with direction. The presenceof the shielding plate 110 directs ion flux with respect to thesubstrate surface 130. In one embodiment, plasma-enhanced chemical vapordeposition (PECVD) is performed using the plate. In another embodiment,SiO₂ etching is performed.

[0023] Since solid objects are plasma density sinks, to minimize thesignature effect of the support 120, the distance D between the supportand the reticle may be made much larger than mean free path of theetching particles, primarily electrons and ions. The mean free path isthe distance that an ion, electron or atom travels without collision. Inone embodiment, the distance between the support and the reticle is 2-3times greater than the mean free path. It is also preferable for thediameter of the support to be much smaller than mean free path.

[0024]FIG. 2 is an illustration of a top-view of two embodiments ofshielding plates, in accordance with the present invention. In oneembodiment, shielding plates are circular. The size, shape and geometryof the shielding plate is sufficient to cause ion flux from plasma toreticle to be diverted in some way. By diverting ion flux, etch ratescan be controlled. In one embodiment, the shielding disk is placed overthe center of the reticle to reduce the higher etch rates that normallyoccur at the reticle center and provide for more uniform etching. Inanother embodiment, the disk is placed above one side of the reticle toprovide for decreased etch rates in that side area.

[0025] Shielding plate 210 is a solid circle which may reduce etch ratesin the central area of reticle 130 (FIG. 1). In another embodiment,shielding plate 220 has a radial cutout to suppress etch rates in theedge area of the reticle 130 (FIG. 1). A cutout is perforation in ashielding plate. This can achieve axisymmetric etch rate adjustment. Inanother embodiment, non-radial perforations may be made in the plate toconcentrate the etching within a certain area of the reticle.

[0026] The diameter and thickness of a plate may depend upon the size ofthe plasma chamber and the reticle. In one embodiment, the dimensions ofthe plate are dependent upon the dimensions of the chamber and reticle.For example, the plate fractionally smaller than the reticle. In oneembodiment, the plate has a thickness of 2-5 mm. It is preferable forthe plate to have rounded corners on its edges to reduce the risk oflocal electrostatic discharge. In one embodiment, the plate and anysupporting members are made of a dielectric material, such as but notlimited to quartz or a ceramic. A dielectric material is an insulatorthrough which electrical current cannot flow.

[0027]FIGS. 3, 4 and 5 demonstrate how numerical simulation can be usedto optimize disk properties. In one embodiment, disk optimization iscarried out by numerical simulation.

[0028]FIG. 3 is an illustration of a cross-section of plasma densitiesfor various embodiments of plate usage, in accordance with the presentinvention. Plasma density is synonymous to electron density. Plasmadensities are measured in number of ions/cm³ at 4 mTorr Ar and 600 W ofICP radio frequency power. In the case A embodiment, there was no diskwas in the plasma chamber. In the case B embodiment, a 2-cm radius diskwas placed at a height of 5 cm above the reticle. In the case Cembodiment, a 4-cm radius disk was placed at 5 cm above the reticle. Inthe case D embodiment, a 4-cm radius disk was placed at 11 cm above thereticle. In the case E embodiment, a hollow disk, or disk with a radialcutout, having a 10-cm outer radius and a 5-cm inner radius was placedat a height of 5 cm above the reticle. The projected density contourlines changed significantly in situations where a disk was present inthe chamber. In embodiments B, C, and D where solid disks were used,plasma density in the central area of the reticle was suppressed. In thecase E embodiment, a hollow disk suppressed the density at the outeredges of the reticle, because plasma can penetrate the hole in the disk.In yet another embodiment, the location of the disk is altered dependingupon changes in chamber pressure or other variables.

[0029]FIG. 4 is an illustration of radial ion flux rates for the variousembodiments of plate usage in FIG. 3, in accordance with the presentinvention. Ion flux is equal to plasma density multiplied by ionvelocity. Ion fluxes across the reticle are measured in terms of a deltavalue which is equal to the =(maximum ion flux minimum ionflux)/(average ion flux). The case A embodiment again represents theconventional approach without a disk in plasma. In the embodiments ofcases A-E, the disks are of the same size, geometry and location as thecorresponding disks in FIG. 3. The embodiment of case D, a 4 cm radiusdisk placed at a height of 11 cm above the reticle, produced the bestuniformity of ion flux across the surface of the reticle and thereforethe most uniform etch rate. The delta value of case D was only 1.5%, ascompared to 5% in the Case A embodiment, where no shielding disk waspresent.

[0030]FIG. 5 is an illustration of a cross-section of plasma potentialsfor the various embodiments of plate usage in FIG. 3, in accordance withthe present invention. Plasma potential are measured at 4 mTorr Ar 600 WICP power. Plasma potential provides that when ion flux emanates fromplasma to the reticle, the ions obtain energy from the surroundingelectrical field. This energy is proportional to the potentialdifference between the reticle and the plasma. With increased potentialand RF bias, ion bombardment upon the reticle becomes more energetic.Ion bombardment is therefore proportional to the potential differencebetween the reticle and the plasma.

[0031] Five embodiments, cases A-E, are represented, each with plategeometry, size and location corresponding to that in FIG. 3. To achieveuniform ion bombardment it is desirable that potential contour lines beas close to parallel to the plane of the reticle as possible. The case Dembodiment therefore provides the most uniform etching because itspotential contour lines are nearly parallel to the reticle.

[0032] Etching is primarily controlled by two factors, 1) ion andetchant fluxes, and 2) energy flux. Energy flux equals mass fluxmultiplied by the energy carried by each ion. Mass flux is the quantityof mass diffusing through and perpendicular to a unit cross-sectionalarea per unit time. Energy is proportional to plasma potential. In orderto achieve uniform etching, it is necessary to achieve uniform ion andactive neutral fluxes as well as uniform energy flux.

[0033]FIG. 6 is an illustration of a cross-section of one embodiment ofa critical dimension control process, in accordance with the presentinvention. Vertical etching 620 of the reticle 640 is carried out byions (i) and metastable neutrons (n*). Horizontal etching 630, however,can be carried out only by neutrons (n*). Shielding disks may be used toregulate the n*/i+n* ratio and control the rates of horizontal andvertical etching. By this means, the desired critical dimensions 610 foran etching process may be generated. Photoresistors or mask 650 on topof reticle 640 prevent etching in the areas of the reticle that are notto be etched. In one embodiment, CD 610 are tailored to fit therequirements of the particular etching process involved.

[0034]FIG. 7 is a flow diagram for one embodiment of a method foroptimizing plasma-assisted material process uniformity, in accordancewith the present invention. At block 705, the optimization processbegins. Optimization is based upon the type of material process to beperformed and the special needs of each process. At block 710, theshielding plate size is optimized by simulation. For example, if theprocess requires that the etching rate be minimized in a relativelysmall area of the reticle, then a smaller disk may be placed above thearea. At block 720, the plate location is optimized by simulation. Forexample, if the process requires that the etching rate over the upperleft hand corner of the reticle be minimized, then a plate can be placedabove the upper left hand corner area. The height of the plate above thereticle may also be optimized to achieve the desired ion flux. At block730, the plate geometry is optimized by simulation. For example, if theprocess requires that the etching rate be minimized around the edges ofthe reticle, then a hollow disk, or disk with a cutout, can be used. Inone embodiment, optimizing plate geometry includes making one or moreperforations in the plate for localized etching. In general, thepresence of a plate above an area of substrate decreases the etch ratein that area. In one embodiment, the optimizations in blocks 710, 720,and 730 are carried out by numerical simulation. At block 740, adetermination is made of whether the plate size, location, and geometryprovide for the optimal ion flux distribution across the substratesurface and/or whether the macro-loading effect has been minimized. Themacro-loading effect is caused by the non-uniform accumulation ofetching by-products across the substrate. For example, in oneembodiment, etching by-products are more heavily concentrated at thecenter of the substrate. This causes the etching rate to be decreased atthe center. A shielding plate may be placed above the substrate centerto prevent ion flux to the center and spread the etching by-productsuniformly across the substrate. If the parameters are not optimal atblock 740, the process returns to block 710 for further optimization.When plate size, location, and geometry have been optimized, the plateis inserted into the plasma chamber at block 750. The presence of theplate diverts ion flux from areas of the substrate beneath the plate. Atblock 760, the desired plasma-enhanced material process is carried out.At block 765, the process ends.

[0035]FIG. 8 is a flow diagram for one embodiment of a method foroptimizing a critical dimension control process, in accordance with thepresent invention. At block 805, the process begins. At block 810, theplate size, location, and geometry are optimized as in FIG. 7. At block820, the ratio of (neutrons)/(neutrons+ions) bombarding the plate isregulated by the presence of the plate. For example, the plate canreduce the ion flux in a certain area of the substrate and therebyincrease the relative number of neutrons in that area. This, in turnincreases the rate of horizontal etching upon the substrate and widensthe critical dimensions of the etched cavity. For example, in a chromiumetching process, where etching rates are higher at the reticle center,producing a deeper cavity than at the reticle sides, a plate could beplaced over the center of the reticle to increase horizontal etchingrates. By this, the different cavities across the substrate would have amore uniform width. At block 830, the rates of horizontal and verticaletching are controlled. At block 840, a determination is made of whetherthe cavities produced have the desired critical dimensions. If not, theprocess returns to block 810 to optimize plate size, location, andgeometry. If the critical dimension process is optimized at block 840,then the process proceeds to block 850. In one embodiment, blocks 820and 830 are carried out by simulation. At block 850, the etching processis carried out. At block 855, the process ends.

[0036] It will be further appreciated that the instructions representedby the blocks in FIGS. 7 and 8 are not required to be performed in theorder illustrated, and that all the processing represented by the blocksmay not be necessary to practice the invention.

[0037] In the foregoing specification, the invention has been describedwith reference to specific exemplary embodiments thereof. It will beevident that various modifications may be made thereto without departingfrom the broader spirit and scope of the invention as set forth in thefollowing claims. The specification and drawings are, accordingly, to beregarded in an illustrative sense rather than a restrictive sense.

What is claimed is:
 1. An apparatus comprising: a plasma chambercontaining a plasma for a plasma-assisted material process upon asubstrate; a shielding plate within said plasma chamber to activelydirect ion flux to desired areas of the substrate; and a supportingstructure to support said shielding plate within said chamber.
 2. Theapparatus of claim 1 wherein the plasma-assisted material process is aplasma-assisted etching process.
 3. The apparatus of claim 1 wherein theplasma-assisted material process is a plasma-enhanced chemical vapordeposition process.
 4. The apparatus of claim 1 wherein the shieldingplate and the supporting structure are composed of a dielectricmaterial.
 5. The apparatus of claim 1 wherein the supporting structurefurther comprises three or more supporting members.
 6. The apparatus ofclaim 1 wherein the shielding plate is solid to suppress ion flux at thecenter of the substrate.
 7. The apparatus of claim 1 wherein theshielding plate has one or more perforations that allow ion flux topass, such that the ion flux within a localized area of the substrate isfitted to meet the requirements of a desired material process.
 8. Theapparatus of claim 1 wherein the dimensions of the plate are dependentupon the dimensions of the plasma chamber and the substrate.
 9. Theapparatus of claim 8 wherein the thickness of the plate is 2-5 mm. 10.The apparatus of claim 1 wherein the distance between a member of saidsupporting structure and said substrate is greater than the mean freepath of a reactive particle.
 11. The apparatus of claim 1 wherein thewidth of a member of said supporting plate is less than the mean freepath of a reactive particle.
 12. The apparatus of claim 1 wherein theedge of said plate is rounded.
 13. The apparatus of claim 1 wherein theplate is circular.
 14. The apparatus of claim 1 wherein theplasma-assisted material process is carried out in high-density plasma.15. A method comprising: optimizing the dimensions, geometry, andlocation of a shielding plate to generate a desired ion flux in aplasma-assisted material process conducted in a plasma chamber;inserting the plate above a substrate in the chamber; and carrying outthe desired material process upon the substrate by the ion fluxgenerated.
 16. The method of claim 15 further comprising optimizing thedimensions, geometry, and location of the shielding plate by numericalsimulation.
 17. The method of claim 16 further comprising performing theoptimization process such that a set of numerically simulated plasmapotential contour lines are as close to parallel to the plane of asimulated substrate surface as possible.
 18. The method of claim 15further comprising varying localized ion flux across said substrate byperforating said plate.
 19. The method of claim 14 further comprisingoptimizing the uniformity of energy flux across the substrate surface.20. A method comprising: actively directing ion flux within a plasmachamber by the insertion of a plate into the chamber; and regulating ionflux to different areas of the substrate by altering properties of theplate.
 21. The method of claim 20 further comprising conducting aplasma-assisted etching process upon the substrate.
 22. The method ofclaim 20 further comprising conducting a plasma-enhanced chemical vapordeposition process upon the substrate.
 23. A method comprising: placinga shielding plate within a plasma chamber to actively direct ion flux,such that the ratio of (neutrons)/(neutrons+ions) bombarding a substrateis regulated.
 24. The method of claim 23 further comprising controllingthe rates of horizontal and vertical etching upon the substrate.
 25. Themethod of claim 24 further comprising producing cavities in thesubstrate having the desired critical dimensions by the directed ionflux.
 26. The method of claim 25 further comprising customizing thedimensions of each cavity according to the requirements of aplasma-assisted etching process.
 27. A method comprising: activelydirecting ion flux within a plasma chamber by the insertion of ashielding plate such that the accumulation of etching by-products acrossthe surface of a substrate is regulated.
 28. The method of claim 27further comprising improving etch uniformity across the substrate. 29.The method of claim 27 further comprising: preventing the non-uniformaccumulation of etching by-products at the center of a substrate; andincreasing the etching rate at the center of the substrate.